Himalayan-Tibetan Plateau Research Articles

Cyclicities and clusters in the lacustrine sequence of Heqing basin (SW China): Its use for dating and palaeoenvironmental reconstruction

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp.145

Characteristics of Clay Minerals in the Northern South China Sea and Its Implications for Evolution of East Asian Monsoon since Miocene

Clay mineral assemblages, crystallinity, chemistry, and micromorphology of clay particles in sediments from ODP Site 1146 in the northern South China Sea (SCS) were analyzed, and used to trace sediment sources and obtain proxy records of the past changes in the East Asian monsoon climate since the Miocene, based on a multi-approach, including X-ray diffraction (XRD) and scanning electron microscopy combined with energy dispersive X-ray spectrometry (SEM-EDS). Clay minerals consist mainly of illite and smectite, with associated chlorite and kaolinite. The illite at ODP Site 1146 has very well-to-well crystallinity, and smectite has moderate-to-poor crystallinity. In SEM the smectite particles at ODP Site 1146 often appear cauliflower-like, a typical micromorphology of volcanic smecites. The smectite at ODP Site 1146 is relatively rich in Si element, but poor in Fe, very similar to the smectite from the West Philippine Sea. In contrast, the chemical composition of illite at ODP Site 1146 has no obvious differences from those of the Loess plateau, Yellow River, Yangtze River, and Pearl River. A further study on sediment source indicates that smectite originates mainly from Luzon, kaolinite from the Pearl River, and illite and chlorite from the Pearl River, Taiwan and/or the Yangtze River. The clay mineral assemblages at ODP Site 1146 were not only controlled by continental eathering regimes surrounding the SCS, but also by the changing strength of the transport processes. The ratios of (illite+chlorite)/smectite at ODP Site 1146 were adopted as proxies for the East Asian monsoon evolution. Relatively higher ratios reflect strongly intensified winter monsoon relative to summer monsoon, in contrast, lower ratios indicate a strengthened summer monsoon relative to winter monsoon. The consistent variation of this clay proxy from those of Loess plateau, eolian deposition in the North Pacific, planktonic, benthic foraminifera, and black carbon in the SCS since 20 Ma shows that three profound shifts of the East Asian winter monsoon intensity, and aridity in the Asian inland and the intensity of winter monsoon relative to summer monsoon, occurred at about 15 Ma, 8 Ma, and the younger at about 3 Ma. The phased uplift of the Himalaya-Tibetan plateau may have played a significant role in strengthening the Asian monsoon at 15 Ma, 8 Ma, and 3 Ma.

Grain-size records at ODP site 1146 from the northern South China Sea: Implications on the East Asian monsoon evolution since 20 Ma

273 samples from Ocean Drilling Program (ODP) Site 1146 in the northern South China Sea (SCS) were analyzed for grain-size distributions using grain-size class vs. standard deviation method and end-member modeling algorithm (EMMA) in order to investigate the evolution of the East Asian monsoon since about 20 Ma. 10–19 μm/1.3–2.4 μm, the ratio of two grain-size populations with the highest variability through time was used to indicate East Asian winter monsoon intensity relative to summer monsoon. The mass accumulation rate of the coarsest end member EM1 (eolian), resulting from EMMA, can be used as a proxy of winter monsoon strength and Asian inland aridity, and the ratio of EM1/(EM2+EM3) as a proxy of winter monsoon intensity relative to summer monsoon. The combined proxies show that a profound enhancement of East Asian winter monsoon strength and winter monsoon intensity relative to summer monsoon occurred at about 8 Ma, and it is possible that the summer monsoon simultaneously intensified with winter monsoon at 3 Ma. Our results are well consistent with the previous studies in loess, eolian deposion in the Pacifc, radiolarians and planktonic foraminifera in the SCS. The phased uplift of the Himalaya-Tibetan Plateau may have played a significant role in strengthening the Asian monsoon at 8 Ma and 3 Ma.

Development of the East Asian monsoon: Mineralogical and sedimentologic records in the northern South China Sea since 20 Ma

We here reconstruct the past change of the East Asian monsoon since 20 Ma using samples from Ocean Drilling Program (ODP) Site 1146 in the northern South China Sea based on a multi-proxy approach including a monomineralic quartz isolation procedure, identification of clay minerals by X-ray Diffraction (XRD) and grain-size analysis of isolated terrigenous materials. Terrigenous supply to ODP Site 1146 was dominated by changes in the strength of multiple sources and transport processes. Grain-size data modeled by an end-member modeling algorithm indicate that eolian dust from the arid Asian inland and fluvial input have contributed on average 20% and 80% of total terrigenous material to ODP Site 1146, respectively. Specifically, about 40–53% of the total (quartz + feldspar) and only 6–11% of the total clay is related to eolian supply at the study site. Detailed analysis of the sedimentary environment, and clay minerals combined with previous studies shows that smectite originates mainly from Luzon, kaolinite from the Pearl River and illite and chlorite from the Pearl River, Taiwan and/or the Yangtze River. The proportion and mass accumulation rate (MAR) of the coarsest end-member EM1 (interpreted as eolian dust), ratios of (illite + chlorite)/smectite, (quartz + feldspar)% and mean grain-size of terrigenous materials at ODP Site 1146 were adopted as proxies for East Asian monsoon evolution. The consistent variation of these independent proxies since 20 Ma shows three profound shifts in the intensity of East Asian winter monsoon relative to summer monsoon, as well as aridity of the Asian continent, occurred at ∼ 15 Ma, ∼ 8 Ma and the youngest at about 3 Ma. In comparison, the summer monsoon intensified contemporaneously with the winter monsoon at 3 Ma. The phased uplift of the Himalaya–Tibetan plateau may have played a significant role in strengthening the Asian monsoon at ∼ 15 Ma, 8 Ma and 3 Ma.

Impacts of tectonic changes on the reorganization of the Cenozoic paleoclimatic patterns in China

Geologic studies have illustrated that the planetary-wind-dominant climate in the Paleogene is changed into the monsoon-dominant one near the Oligocene/Miocene boundary in China. The evolution is marked by the changes of regional aridity/humidity contrasts. The contrasts occur between the south and the north part of China in the Eocene, and then between East China and Central Asia near the Oligocene/Miocene boundary, indicating the onset of monsoon-dominant climate in China. The impacts of the Himalaya–Tibetan plateau uplift and/or the Paratethys Sea retreat on the Asian monsoon have been well demonstrated. However, whether or not other factors have affected the above reorganization of paleoclimatic patterns remains a question to be addressed. Additional factors that should be addressed at least include the Indian Peninsular drift, the South China Sea expansion and the East China Sea transgression. Here we use the IAP-AGCM to explore their roles in the above paleoclimatic evolution. Our experiments demonstrate that the South China Sea expansion is another major forcing, in addition to the important roles of the Paratethys retreat and the Himalaya–Tibetan plateau uplift. On the contrary the impacts of the Indian Peninsular drift and the East China Sea transgression are relatively subordinate. The Himalaya–Tibetan plateau uplift plays a crucial role in the magnification of the aridity/humidity contrasts between the south and the north part of China. The Paratethys retreat, the Himalaya–Tibetan plateau uplift and the South China Sea expansion coact to cause the formation of the aridity/humidity contrasts between East China and Central Asia. The retreat and the uplift favor the dynamic condition, and the expansion provides the water vapor condition for the monsoon-dominant climate in China.

Himalayan evolutionary models: Constraints from Eocene–Miocene granitoid bodies

Orogenic reconstructions depend on adequate knowledge of the spatial and temporal associations between processes, structures and lithotectonic units. However, the present gamut of Himalayan evolutionary models may be biased by the paucity of data from the ca. 55 to 25 Ma period following the initiation of collision, the lack of constraints on the structural assembly of many parts of the orogenic zone, and traditional focus on the frontal parts of the High Himalaya. The record of granitoid magmatism preserved in the eastern North Himalaya and southern Tibetan plateau places important new constraints on the time-dependent structural architecture and locus of tectonic activity in the Himalaya since at least the mid-Eocene. Combined geochronological, geochemical and thermometric studies of undeformed Eocene granitoid plutons emplaced into Tethyan metasediments at 92 E (the Dala granites) constrain the timing of deformation in the structurally highest (and inferred earliest accreted units of the Himalayan fold and thrust belt) to be older than 45 Ma. A similar study of nearby Miocene granites (the Yala-Xiangbo granites) provides further insight into subsequent reorganisation of the Himalayan thrust system and the formation of the north Himalayan gneiss domes (NHG). Petrogenesis of both suites, combined with documented evidence for essentially continuous arc-type magmatism along the southern margin of Eurasia, argue strongly against underthrusting of Indian lithosphere beneath southern Tibet, either as a means of thickening the crust or providing insulation to promote development of a weak mid-crustal layer. This study demonstrates the integrated application of geochemistry, geochronology and structural geology in probing the time-dependent structural evolution of an active orogenic zone.

About this title

This collection of 27 review and research papers provides an overview of the geodynamic concepts of channel flow and ductile extrusion in continental collision zones. The focal point tor this volume is the proposal that the middle or lower crust acts as a ductile, partially molten channel flowing out from beneath areas of over-thickened crust, such as the Tibetan plateau, towards the topographic surface at plateau margins. This controversial proposal explains many features related to the geodynamic evolution of the plateau and, for example, extrusion and exhumation of the crystalline core of the Himalayan mountain chain to the south. In this volume thermal-mechanical models for channel flow, extrusion and exhumation are presented, and geological and geophysical evidence both for and against the applicability of such models to the Himalayan-Tibetan Plateau system, as well as older continental collision zones such as the Hellenides, the Appalachians and the Canadian Cordillera, are discussed.

A 25 m.y. isotopic record of paleodiet and environmental change from fossil mammals and paleosols from the NE margin of the Tibetan Plateau

We use the carbon and oxygen isotopic compositions of fossil tooth enamel and paleosols to reconstruct the late Cenozoic history of vegetation and environmental change in the Linxia Basin at the northeastern margin of the Tibetan Plateau. The δ 13C values of fossil enamel from a diverse group of herbivores and of paleosol carbonate and organic matter indicate that C4 grasses were either absent or insignificant in the Linxia Basin prior to ∼2–3 Ma and only became a significant component of local ecosystems in the Quaternary. This is in striking contrast to what was observed in Pakistan, Nepal, Africa and the Americas where C4 plants expanded rapidly in the late Miocene as indicated by a positive δ 13C shift in mammalian tooth enamel and paleosols. The δ 18O results from the same herbivore species show several significant shifts in climate in the late Cenozoic. Most notably, a positive δ 18O shift after ∼7 Ma indicates a shift to warmer and/or drier conditions and is comparable in timing and direction to the δ 18O shift observed in paleosol carbonates in Pakistan and Nepal. This late Miocene climate change observed in the Indian sub-continent and in the Linxia Basin, however, seems to be a regional manifestation of a global climate change. The lack of evidence for C4 plants in the Linxia Basin prior to ∼2–3 Ma suggests that the East Asian summer monsoon, which brings precipitation into northern China during the summer and creates optimal conditions for the growth of C4 grasses, was probably not strong enough to affect this part of China throughout much of the Neogene. This implies that the Himalayan-Tibetan Plateau may not have reached the present-day elevation across its vast extent to support a strong East Asian monsoon system before ∼2–3 Ma. Our data also suggest that regional climatic conditions played an important role in controlling the expansion of C4 plants.

Late Oligocene rapid transformations in the South China Sea

Lithobiostratigraphic data indicate that the double reflectors on the seismic profile through Ocean Drilling Program (ODP) Site 1148 represent two unconformities that coincide, respectively, with the lower/upper Oligocene boundary at ∼488 mcd, and Oligocene–Miocene boundary at 460 mcd. Two other unconformities, at ∼478 and 472 mcd, respectively, were also identified within the upper Oligocene section. Together they erased a sediment record of about 3 Ma from this locality in a period of very active seafloor spreading. The existence of 32.8 Ma marine sediment at the terminated depth (850 mcd) indicates that the initial breakup of the South China Sea (SCS) was probably during 34–33 Ma, close to the Eocene–Oligocene boundary. High sedimentation rates of 60–115 m/my from the much expanded, >350 m lower Oligocene section resulted from rifting and rapid subsidence between 33 and 29 Ma. The mid-Oligocene unconformity at ∼28.5 Ma, which also occurred in many parts of the Indo-West Pacific region, was probably related to a significant uplift of the Himalayan-Tibetan Plateau to the west and the initial collision between Indonesia and Australia in the south. A narrowed Indonesian seaway may have accounted for the late Oligocene warming and chalk deposition in the northern South China Sea including the Site 1148 locality. The unconformities and slumps near the Oligocene–Miocene boundary indicate a very unstable tectonic regime, probably corresponding to changes in the rotation of different land blocks and the seafloor spreading ridge from nearly E–W to NE–SW, as recognized earlier at magnetic Anomaly 7. This 25 Ma event also saw the first New Guinea terrane docking at the northern Australian craton. The low sedimentation rate of ∼15 m/my in the early to middle Miocene may correspond to another period of rapid seafloor spreading and rapid widespread subsidence that effectively caused sediment source areas to retreat with a rapidly rising sea level. The isostatic nature of these late Oligocene unconformities and slumps with several major collision-uplift events indicate that the rapid changes in the early evolutionary history of the South China Sea were mainly responding to regional tectonic reconfiguration including the uplift-driven southeast extrusion of the Indochina subcontinent.

Comparison of simulated changes of climate in Asia for two scenarios: Early Miocene to present, and present to future enhanced greenhouse

We use a climate model (GENESIS) to simulate the changes in climate associated with two scenarios, one from the past and one from the future, with a focus on the Asian continent. The two scenarios are: (1) Early Miocene to Present—a period of uplift of the Himalayan–Tibetan plateau and of decreasing concentration of atmospheric carbon dioxide, and (2) Present to Future Enhanced Greenhouse—a period of increasing concentration of atmospheric carbon dioxide. In the past climate scenario, the combination of uplift and decreased concentration of greenhouse gas causes the model to simulate widespread cooling and, primarily due to the effect of uplift, greatly increased precipitation in southern Asia and decreased precipitation in northern Asia. In the future climate scenario, the increased concentration of atmospheric carbon dioxide causes the model to simulate widespread warming and, by comparison with the past climate scenario, relatively small changes in precipitation; the changes are generally towards increased precipitation, except in parts of northern China. The output of the climate model, along with the changed concentration of atmospheric carbon dioxide, is also used to calculate changes in biome distributions. Owing to the high concentrations of atmospheric carbon dioxide in both the past and future scenarios, relative to present, the simulations of Early Miocene biomes and Future biomes are somewhat similar—and both are very unlike the Present.

Fe–Al-rich tridymite–hercynite xenoliths with positive cerium anomalies: preserved lateritic paleosols and implications for Miocene climate

We report isotopic and chemical compositions of unusual tridymite–hercynite xenoliths in middle Miocene Niutoushan tholeiites from the southeast coastal area of China. These xenoliths are characterized by positive cerium (Ce) anomalies and extremely high Al 2O 3 (32–34 wt.%) and total iron oxide (20–22%). They have 87Sr/ 86Sr of 0.7050–0.7058, ε Nd(0) values of +3.2 to +4.2, 206Pb/ 204Pb ratios of 18.8–19.1, and δ 18O values of +5.2‰ to +6.1‰. Their chemical and isotopic compositions suggest that these xenoliths represent preserved aluminous lateritic paleosols that are not genetically related to host tholeiites. These lateritic paleosols with strongly desilicated minerals were formed by intense chemical weathering under warm and humid tropical conditions (with annual average temperature of >19 °C and the annual rainfall of >165 cm) in SE China during the interval from 17 to 15 Ma. The formation age of the paleosols corresponds to a period characterized by slow uplift of the Himalayan–Tibetan Plateau region (and thus less consumption of CO 2) after 17 Ma, and eruptions of 17–15 Ma Columbia River flood basalts, the Vogelsberg basalts, and eastern China basalts (and thus more input of CO 2 into the atmosphere). The tridymite–hercynite xenoliths in the Niutoushan basalts thus preserve evidence of extraordinary climatic greenhouse conditions in the middle Miocene that would otherwise have been lost by the erosion of paleosols.

ヒマラヤチベット高原の隆起, 東アジアモンスーンの発達と東アジア縁海における堆積作用

ヒマラヤチベット高原の隆起, 東アジアモンスーンの発達と東アジア縁海における堆積作用

Re-Os isotope systematics of sediments of the Brahmaputra River system

Re-Os analyses were performed on suspended loads and coarser grained bank sediments of the Brahmaputra River system. Re and Os concentrations of these sediments vary from 7 to 1154 ppt and from 3 to 173 ppt, respectively. 187Os/ 188Os ratios range from 0.178 to 6.8, and thus vary from nearly mantle to very radiogenic crustal values. Nevertheless, most of the sediments have 187Os/ 188Os ratios less than 1.5, and nearly all of the samples of the Brahmaputra main channel have ratios less than 1.2. Thus, as previously suggested, the Brahmaputra is much less radiogenic than the Ganga. The Siang River, the northern extension of the Brahmaputra, is quite radiogenic in Os despite receiving sediments from the Tsangpo River, which flows along a suture zone with ultramafic outcrops. The Brahmaputra main channel has a fairly constant 187Os/ 188Os ratio even though its tributaries contribute sediments with very heterogeneous Os isotopic compositions. These data, along with the corresponding Nd isotopic compositions, suggest that about 60–90% of the sediment in the Brahmaputra system is derived from Himalayan formations (Higher Himalaya and Lesser Himalaya) whereas 10–40% comes from ophiolite-bearing sequences, perhaps eastern equivalents of those of the Transhimalayan Plutonic Belt. Os data also confirm previously published Sr and Nd results, indicating that about half of the sediments delivered to the Brahmaputra are supplied by the Siang River, while the Himalayan and the eastern tributaries account for 40 and 10%, respectively. The lower 187Os/ 188Os of the Brahmaputra River compared to that of the Ganga is due to two factors. One is the more limited presence of the Lesser Himalaya and hence the lower black shale content of the eastern Himalaya. The other is the non-radiogenic Os supplied by the eastern and southern tributaries, reflecting the presence of mantle-derived lithologies in this region. Despite the lower sediment supply from these tributaries, they contribute greatly to the Os budget of the Brahmaputra River. This study indicates that the Brahmaputra River has little effect on the present-day seawater Os budget. However, reconsideration of this budget suggests that the Ganga, which provides the most radiogenic Os of major rivers studied to date, may have significant impact on the marine Os isotopic composition. The Indo-Asian collision cannot be excluded as an important cause of the increase in the marine 187Os/ 188Os over the past 16 million years until the contributions of all of the rivers draining the Himalayan Tibetan Plateau are known.

Southern migration of westerlies in the Northern Hemisphere PEP II transect during the Last Glacial Maximum

A southward shift of continuous and discontinuous permafrost zones and vegetation belts of Taiga, broad-leaf deciduous and evergreen forests on the islands forming Japan suggests a southern migration of the Northern Hemisphere (NH) summer westerly subtropical jet by around 3–5° of latitude at 135–140°E during the global Last Glacial Maximum (LGM). Since this jet induces the Asian (NH) summer monsoon, the southern shift caused a cooler and drier summer, and the spread of deciduous forest in Southern Japan. The expansion of mountain glaciers on the islands of Japan also supports the southern migration of the subtropical westerly jet. At present, the ablation of snow occurs by heat and precipitation carried by the NH summer monsoon. By comparison, the position of the NH winter westerly subtropical jet remained unchanged, presumably because its course was constrained by the Himalayan–Tibetan Plateau. The northern limit of the Kuroshio Current, corresponding to the boundary of the subtropical and subarctic water in the northwestern Pacific Ocean, also migrated by around 3° during the global LGM. The southern migration of the westerly subtropical jet suggests that the current situation where its location shifts during the NH summer to north of the Tibetan Plateau did not occur during the coldest phases of the global LGM, although it did occur intermittently during Marine Isotope Stage (MIS) 3.

Evolution of Asian monsoons and phased uplift of the Himalaya-Tibetan plateau since Late Miocene times.

The climates of Asia are affected significantly by the extent and height of the Himalayan mountains and the Tibetan plateau. Uplift of this region began about 50 Myr ago, and further significant increases in altitude of the Tibetan plateau are thought to have occurred about 10-8 Myr ago, or more recently. However, the climatic consequences of this uplift remain unclear. Here we use records of aeolian sediments from China and marine sediments from the Indian and North Pacific oceans to identify three stages of evolution of Asian climates: first, enhanced aridity in the Asian interior and onset of the Indian and east Asian monsoons, about 9-8 Myr ago; next, continued intensification of the east Asian summer and winter monsoons, together with increased dust transport to the North Pacific Ocean, about 3.6-2.6 Myr ago; and last, increased variability and possible weakening of the Indian and east Asian summer monsoons and continued strengthening of the east Asian winter monsoon since about 2.6 Myr ago. The results of a numerical climate-model experiment, using idealized stepwise increases of mountain-plateau elevation, support the argument that the stages in evolution of Asian monsoons are linked to phases of Himalaya-Tibetan plateau uplift and to Northern Hemisphere glaciation.

QUATERNARY PALYNOFLORAS AND PALEOCLIMATE OF THE QAIDAM BASIN, QINGHAI PROVINCE, NORTHWESTERN CHINA

A cool, arid climate featuring steppe vegetation characterizes the modern day temperate zone of northwestern China. In contrast, palynofloras indicate that the paleoclimate there was warmer and wetter during the Pleistocene than during the Holocene. To document vegetational and climatic changes during the Quaternary, fossil pollen and spores were systematically studied in sediments from the Qaidam Basin in the Qinghai Province at the northeastern margin of the Tibetan (Qinghai–Xizang) Plateau. Pollen and spores in four cores from the Quaternary lacustrine deposits in the Qaidam Basin showed four distinctive pollen zones. The basal assemblage, Zone Q1, is dominated by taxa having tropical or subtropical warm, wet climatic affinity and is probably Early Pleistocene in age. The overlying assemblage, Zone Q2, is dominated by taxa having warm–temperate and semi–wet climatic affinity and is probably Middle Pleistocene in age. Zone Q3 is dominated by taxa having temperate and semiarid climatic affinity and is probably Late Pleistocene in age. The uppermost assemblage, Zone Q1, is dominated by taxa having cool–temperate, arid climatic affinity and is Holocene in age. The modern vegetation of the high plateau is dominated by xerophytic and halophytic herbs and shrubs. The palynofloral assemblages show that this modern vegetation became established during the Holocene, when cool, dry conditions prevailed following Pleistocene deglaciation. This climatic cooling is interpreted as the result of continued Himalayan–Tibetan Plateau uplift in the Holocene. Modern vegetation zones are used as a basis for comparison with the fossil assemblages and suggest that the Qaidam Basin might have been elevated at least 2,000 to 3,000 m since the Early Pleistocene. Dabuxun Lake in the basin may have been elevated about 700 m in the past half million years. The pollen data therefore allow more precise dating of Himalayan–Tibetan uplift. Intense uplift at the end of Early Pleistocene is indicated and further uplift probably occurred in the middle substage of the Middle Pleistocene. The results of this study contribute to understanding Himalayan–Tibetan Plateau evolution, regional Quaternary correlations, and climatic changes around the globe.

Re-Os isotope systematics in black shales from the Lesser Himalaya: their chronology and role in the 187Os/ 188Os evolution of seawater

Re and Os abundances and Os isotope systematics have been measured in a number of black shales sampled from outcrops and two underground phosphorite mines, Maldeota and Durmala, in the Lesser Himalaya. The Re and Os concentrations in these samples exhibit wide range, 0.2 to 264 ng/g and 0.02 to 13.5 ng/g, respectively, with a mean Re/Os of ∼25. The 187Os/ 188Os of these samples also show a broad range 1.02 to 11.6, with many of them far more radiogenic than the currently eroding continental crust. The black shales from the Maldeota and Durmala mines, collected a few meters above the Krol-Tal (Pc-C) boundary and occurring in the same stratigraphic horizon, yield 187Re– 187Os isochron ages of 554 ± 16 and 552 ± 22 Ma, respectively, consistent with fossil evidences and those assigned for the Pc-C boundary at various other locations. These results indicate closed system behaviour of Re and Os in these mine samples and reaffirm the validity of 187Re– 187Os pair to date organic-rich sediments. The age of these mine samples from the outer belt seems to be a few hundred millions of years younger than the preliminary Re-Os age of 839 ± 138 Ma for black shales from Theog, collected from the inner belt. These initial results lean toward the hypothesis that the inner belt sediments were deposited earlier than the outer belt; however, more analyses of black shales are needed to confirm this. The role of weathering black shales from the Himalaya in contributing to the Os isotope evolution of seawater over the past ∼25 Ma was assessed using a simple budget model for seawater Os and assuming values for Os fluxes and 187Os/ 188Os based on the available limited data on Os concentration and its 187Os/ 188Os in rivers and oceans. These calculations indicate that the required contributions from black shale weathering to account for the entire increase in oceanic 187Os/ 188Os over the past ∼16 Ma is difficult to satisfy by varying only the Os flux from HTP rivers ( 187Os/ 188Os kept constant at 1.324) or their 187Os/ 188Os ( 188Os flux constant at today’s value, ∼131 moles/yr). The requirements to balance Os isotope budget in ocean through weathering of black shales, however, is easier to accommodate if both the Os flux from HTP (Himalayan-Tibetan Plateau) rivers and their 187Os/ 188Os are allowed to vary. Measurements of Os abundance in HTP rivers and its isotopic composition are needed to test the validity of the model results.

Carbon cycle and climate change during the Cretaceous inferred from a biogeochemical carbon cycle model

The carbon cycle and climate change during the Cretaceous are reconstructed by using a carbon cycle model, and discussed. The model takes into account the effects of the enhanced magma eruption and organic carbon burial rates, both of which characterize the carbon cycle during the Cretaceous. The result for the CO2 variation is roughly consistent with the pattern of paleoclimate change inferred from the geological record. The CO2 level during the mid‐Cretaceous is estimated to be 4–5 times the present atmospheric level, corresponding to a surface temperature of 20–21°C. The warm, equable Cretaceous resulted from the effects of tectonic forcing such as enhanced CO2 degassing, although the enhanced organic carbon burial has a tendency to decrease the CO2 level. The organic carbon burial rate during the Cretaceous is generally larger than those for the Cenozoic, and is characterized by three major peaks (~ 1.5–1.8 times the present‐day value) corresponding to the major oceanic anoxic events. In the case for the extensive mantle plume degassing, although the CO2 levels are only 10% higher than those for the standard case during 120–100 Ma, the causes for the enhanced CO2 levels would be quite different. If the globally averaged surface temperature had increased due to paleogeographic forcing effects, the greenhouse effect of CO2 (and thus the CO2 level) should be lower than the values estimated for the standard case. If the CO2 levels are similar to, but the surface temperature is higher than, those for the standard case, either the parameter β (an influence of the Himalayas–Tibetan Plateau on the global weathering today) may be unreasonably large or the dependence of the silicate weathering rate on the CO2 partial pressure and the surface temperature should be much weaker than those previously proposed.

Lateritic bauxite in central Germany and implications for Miocene palaeoclimate

Intense chemical weathering during middle Miocene times led to the formation of deep ferralitic weathering profiles on the Vogelsberg basaltic massif. Saprolite more than 50 m thick, was formed and, in its higher parts, bauxite concretions developed by absolute accumulation of aluminium. The tops of the profiles consist mostly of allochthonous, lateritic bauxite which has been mined intensively in the past. Similar high latitude bauxites occur on the Paleogene basalts of Ireland. These formed during a phase of recognized greenhouse conditions with a climatic optimum near the Paleocene-Eocene boundary, while during the Miocene the climate was already changing toward Quaternary icehouse conditions. However, the occurrence of deep weathering profiles with lateritic bauxite at the high palaeolatitude of 45°N may be interpreted as pointing to extreme climatic conditions also during the middle Miocene. The associated laterites and bauxites on middle Miocene basalts (17-15 Ma) provide evidence for a global climatic optimum around 17-14.5 Ma. Reduced uplift rates of the Himalayan-Tibetan Plateau and associated reduced CO2 consumption, coupled with additional volcanogenic CO2 production could have induced high atmospheric CO2 and an intensification of the global greenhouse effect. This may be reflected in accelerated chemical weathering and bauxite formation. Basalt volcanism has thus provided both a source (eruption) and a sink (via weathering) of CO2.

地球環境変化とヒマラヤ・チベット山塊の役割 日本海‐黄土地帯‐地中海トランセクトにおける過去２４０万年間の大気循環変動 チベット／ヒマラヤの上昇史との関係

Recent atmospheric circulations over the Asian Continent have been influenced by the Himalayan Range and the Tibetan Plateau as a wind barrier. Based on recent meteorological data, the existence of the Himalaya-Tibet within the troposphere has caused meandering of the Westerlies, intense activity of the Asian Monsoon and appearance of the Easterlies. Meandering of the Westerlies has led to increase precipitation in the East Asia region including northeastern China and Japan, and has formed the soils with poorly crystalized illite minerals in the Chinese Loess Plateau and the Japanese Islands. In nothern India and Nepal including the frontal basin of the Himalayan Range such as the Kathmandu Basin, intense activity of the Asian Monsoon has led abruptly to increase precipitation named “onset of monsoon”. Also, appearance of the Easterlies from the Himalaya-Tibet to northern Africa within the upper part of the troposphere has led to dry and spread the arid area in the northern Africa.There are many sedimentary records indicating Late Quaternary climatic changes around the Himalayan-Tibetan Plateau. We expected to determine the formation age of recent atmospheric circulation. This age indicates when the Himalayan-Tibetan Plateau was abruptly uplifted and elevated. In other words, formation ages of more humid climate in northeastern China, the Japanese Islands and the Kathmandu Basin, and the driest climate in the northern Africa are useful to determine age of uplift and elevation of the Himalayan-Tibetan Plateau.On the base of above-mentioned evidences and hypothesis, we attempted to investigate magnetic susceptibility of the loess-paleosol sequence in the Luochuan area of th e northeastern China, and illite crystallinity of loess-paleosol sequence in the Kathmandu Basi n and deep-sea sediments of the Japan Sea and the Mediterrannean Sea. Because characteristic changes of magnetic susceptibility and illite crystallinity indicate dry -wet oscillations as useful detectors.As results, we clarified three evidences as follows:1) Large amplitudes of magnetic susceptibility oscillations by large pricipitation in interglacial periods appeared in the Luochuan loess -paleosol sequence during last 600ka.2) Large amplitudes of illite crystallinity oscillations by large precipitations caused decomposition of illite minerals appeared in the deep-sea sediments of the Sea of Japan during the last 800ka and in the loess-paleosol sequence of the Kathmandu Basi n during the last 1.1Ma. 3) Amplitudes of illite crystallinity oscillation during the last 800k a were smaller than those amplitudes since 800ka ago.Investigation mentioned above indicates that critical uplift and elevations of the Himalayan-Tibetan Plateau achieved to induce activity of westerlies, easterlies and the Asian Monsoon, and suggests that determined ages of critical elevation caused atmospheric circulation changes range from 0.6 million years to 1.1 million years ago.